Two different techniques have been known so far for determining this contrast agent concentration by means of computed tomography (CT). Thus, it is possible before and after administering contrast agent to record a computed tomography picture of the body area in which the contrast agent concentration is to be measured. After the two CT images obtained thereby are registered, they are subtracted from one another in order to obtain the rise in the x-ray attenuation values for each pixel or each voxel that is caused by the contrast agent. This rise in the x-ray attenuation values is proportional to the concentration of the contrast agent. However, the computed tomography pictures at different times that are required in this case can entail registration and/or movement artifacts that can lead to a defective determination. When use is made of a contrast agent that collects only slowly in the body material, there is a need, in addition, to observe an undesirably long waiting time between the two computed tomography pictures.
The second known technique employs the use of a multi-energy computer tomograph for simultaneously recording two computed tomography pictures with a different spectral distribution of the x-radiation, that is to say a different x-ray energy. In one variant of this technique, the image data records for two x-ray energies are firstly reconstructed separately from one another. Subsequently, the measured x-radiation values for each voxel are decomposed into the molecular density of two base materials (2-material decomposition), of which one base material constitutes the contrast agent. The two equations resulting from the decomposition can then be used to determine for each voxel the two unknowns, the concentrations of the two base materials. However, for many body materials this technique does not yield satisfactory results, since the decomposition is difficult to predict for all the material components contained in the body material. Thus, the application of this technique for determining the contrast agent concentration in the liver, which generally also contains relatively large fractions of fat, leads to a mixture of the two base materials that is difficult to interpret.